1. Field of the Invention
The present invention relates to a piezoelectric device including a piezoelectric single crystal thin film, in particular, a piezoelectric device having a membrane structure, and a method for manufacturing the piezoelectric device.
2. Description of the Related Art
At present, a large number of piezoelectric devices with a thin film of a piezoelectric single crystal have been developed. The piezoelectric devices employing such a piezoelectric thin film require a support member which supports the piezoelectric thin film when actually assembled. Such a support member is disposed on one principal surface of the piezoelectric thin film. In this case, the support member is not disposed in a vibration region, which actually functions as the piezoelectric device, of the piezoelectric thin film, and the support member is disposed only in a non-vibration region, which does not function as the piezoelectric device, of the piezoelectric thin film. This structure is referred to as a so-called membrane structure.
A method for manufacturing an existing piezoelectric device having the membrane structure is described with reference to FIGS. 1A and 1B.
FIGS. 1A and 1B are views schematically illustrating the method for manufacturing an existing piezoelectric device. FIG. 1A is a cross sectional view of the device after performing the steps of forming a composite substrate of a piezoelectric thin film 10 and support members 40 and 50 including a sacrificial layer 30 that is to be a cavity 80 and a lower electrode 20; and forming an upper electrode 60 on the surface of the piezoelectric thin film 10. In this state, the lower electrode 20 and the sacrificial layer 30 are completely covered with the piezoelectric thin film 10 and the support members 40 and 50. Therefore, it is required to form an opening portion 82 through which the lower electrode 20 is exposed to the side of the piezoelectric thin film 10, and a through hole 81 through which a portion of the sacrificial layer 30 is exposed to the side of the piezoelectric thin film 10.
Thus, a lower electrode exposure process is performed by applying a resist film to the top surface of the piezoelectric thin film 10, forming an etching window in a region of the resist film through which the lower electrode 20 is exposed, and etching to thereby form the opening portion 82 in the piezoelectric thin film 10.
Next, a sacrificial layer exposure process is performed by applying a resist film to the top surface of the piezoelectric thin film 10 again, forming an etching window in a region of the resist film through which the sacrificial layer 30 is exposed, and etching to thereby form the through hole 81 in the upper electrode 60, the piezoelectric thin film 10, and the lower electrode 20.
Next, a sacrificial layer removal process for removing the sacrificial layer 30 through the through hole 81 is performed. Thus, the sacrificial layer 30 is formed to be the cavity 80.
Finally, a lead-out wiring 63A is formed between the upper electrode 60 and a bump pad (not illustrated) and a lead-out wiring 63B is formed between the lower electrode 20 and a bump pad (not illustrated).
The etching depth in the lower electrode exposure process described above and the etching depth in the sacrificial layer exposure process described above are different each other. More specifically, it is required in the lower electrode exposure process to perform etching corresponding to the thickness of the piezoelectric thin film 10 in such a manner that the lower electrode 20 is not etched, and it is required in the sacrificial layer exposure process to perform etching corresponding to the thickness of the piezoelectric thin film 10 and the total thickness of the thickness of the upper electrode 60 and the thickness of the lower electrode 20. Therefore, these two etching processes are different in etching depth or etching material. Thus, the etching processes have been carried out in discrete processes.
Japanese Unexamined Patent Application Publication No. 2007-228319 discloses a method for bonding a lower electrode and a piezoelectric substrate to each other to form a piezoelectric thin film on the surface of the lower electrode.
However, in the existing manufacturing method, etching is separately performed in the sacrificial layer exposure process and the lower electrode exposure process, so that the etching is carried out twice in total. Therefore, damage to the piezoelectric thin film 10 caused by the etching processes has been serious. In particular, when dry etching has been performed in the sacrificial layer exposure process and in the lower electrode exposure process, the piezoelectric thin film 10 is electrically charged, so that polarization degradation may occur in the piezoelectric thin film 10 by the charges in some cases.
Moreover, in each process of the sacrificial layer exposure process and the lower electrode exposure process, the time required for the etching process, which includes a resist film applying step, a photolithography step, and an etching step, is long, which is one of the causes of a high manufacturing cost.